From what I know, the Glock standard springs are 18. IMO, I would install a factory or aftermarket stainless spring, in the standard 18 lbs. I would put the correct barrel in and go to the range and fire MANY rounds using the original barrel. You need to rule out the fact that it's a problem with the conversion or the weapon itself. If it does just fine with the .40 barrel, then I would think twice about using the 9mm barrel. By my own practice, I personally don't like the conversions, for any firearm. If I own a .40 and want to fire 9mm, I would buy one. (Gun, not barrel). Just a thought, but someone will probably chime in with another answer. Good Luck!